Protein tyrosine kinases (PTKs) are a family of enzymes, which transfer the γ-phosphate of ATP to the side chain of tyrosine residues on substrate proteins. PTKs are involved in a variety of cellular processes, including signal transduction and growth regulation. Phosphorylation of substrates by PTKs are key events in cellular signaling.
One class of PTKs are receptor tyrosine kinases (RTKs). These kinases belong to a family of transmembrane proteins and have been implicated in cellular signaling pathways. The predominant biological activity of some receptor kinases is the stimulation of cell growth and proliferation, while other receptor tyrosine kinases are involved in inhibiting growth and promoting differentiation. In some instances, a single tyrosine kinase can inhibit, or stimulate, cell proliferation depending on the cellular environment in which it is expressed (Schlessinger and Ullrich, Neuron (1992), 9(3): 383-391). RTKs include receptors for platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), hepatocyte growth factor (HGF), insulin, insulin-like growth factor 1 (IGF-1), nerve growth factor (NGF), vascular endothelial growth factor (VEGF), macrophage colony stimulating factor (M-CSF) and others.
Receptor tyrosine kinases are composed of at least three domains: an extracellular glycosylated ligand binding domain, a transmembrane domain and a cytoplasmic catalytic domain that can phosphorylate tyrosine residues. Binding of a ligand to membrane-bound receptors induces the formation of receptor dimers and allosteric changes thus activating the intercellular kinase domains which further results self-phosphorylation (autophosphorylation and/or transphosphorylation) of the receptor on tyrosine residues. Receptor phosphorylation stimulates physical association of the activated receptor with target molecules. Some of the target molecules are, in turn, phosphorylated, a process which transmits the signal to the cytoplasm. The secondary signal transducer molecules generated by activated receptors, result in a signal cascade that regulates cell functions such as cell division or differentiation. Intracellular signal transduction is reviewed in Aaronson, Science (1991), 254: 1146-1153; Schlessinger, J. Trends Biochem. Sci. (1988), 13: 443-447; Ullrich and Schlessinger, Cell (1990), 61: 203-212.
Various cell proliferative disorders have been associated with defects in pathways mediated by PTKs. Enhanced activities of PTKs resulting from overexpression of the normal kinase, upregulation of ligands of receptor tyrosine kinases or activating mutations, are a hallmark of many diseases which involve cellular proliferation, including cancer. Examples of specific receptor tyrosine kinases associated with cell proliferative disorders include platelet derived growth factor receptor (PDGFR), insulin-like growth factor 1 receptor (IGF-1R), epidermal growth factor receptor (EDFR), and the related HER.
The involvement of PTKs in various diseases identifies them as targets for antiproliferative drugs. Indeed, numerous PTK blockers have been described in the literature including proposed mechanisms of action (Levitzki et al., Science (1995), 267:1782-88; Posner et al., Mol. Pharmacol. (1994), 45:673-683). Applicants have developed a family of PTK inhibitors, named tyrphostins, designed to mimic the tyrosine substrate (Levitzki et al., Science (1995), 267:1782-88; Levitzki et al., Biochem. Pharm. (1990), 40:913-920; Levitzki et al., FASEB J. (1992), 6:3275-3282; U.S. Pat. Nos. 5,217,999 and 5,773,476). The pharmacophores of these tyrphostins, and in particular tyrphostins of the benzylidene malonitril type, are the hydrophilic catechol ring and the more lipophilic substituted cyano-vinyl radical. Kinetic studies have shown that some tyrphostin compounds are pure competitive inhibitors vis-á-vis tyrosine substrates whereas for the ATP binding site they act as non-competitive inhibitors (Yaish et al., Science (1988), 242:933-935; Gazit et al., J. Med. Chem. (1989), 32:2344-2352). Nonetheless, many tyrphostins have shown competitive inhibition against both the substrate and ATP binding site (Posner et al., Mol. Pharmacol. (1994), 45:673-683).
In a related group of tyrphostins, the hydrophilic catechol ring was exchanged by lipophilic dichloro- or dimethoxy-phenyl groups, to yield EGFR kinase inhibitors, effective in the low micromolar range. (Yoneda et al., Cancer Res. (1991), 51: 4430-4435). However, there is an unmet need for tyrphostins with increased inhibitory properties.